Role of ceria in the improvement of SO2 resistance of LaxCe1 - XFeO3 catalysts for catalytic reduction of NO with CO

Article abstract of DOI:10.1016/j.catcom.2016.03.005

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_{1 - x}FeO₃ compositions were prepared by sol-gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_{1 - x}FeO3 composite oxides.

Full text of DOI:10.1016/j.catcom.2016.03.005

Catalysis Communications 79 (2016) 53–57
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Role of ceria in the improvement of SO resistance of La Ce1 − xFeO3
2
x
catalysts for catalytic reduction of NO with CO
Yihong Qin, Liguo Sun ⁎, Danglong Zhang, Lei Huang
School of Metallurgy and Environment, Central South University, Changsha, Hunan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Perovskite-type catalysts with LaFeO3 and substituted La Ce1 − xFeO3 compositions were prepared by sol–gel
method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy
x
Received 28 December 2015
Received in revised form 7 March 2016
Accepted 8 March 2016
(
(
XPS), CO temperature-programmed reduction (CO-TPR), and SO
SO -TPD). Catalytic reaction for NO reduction with CO in the presence of SO
exhibited an excellent catalytic activity without SO , but decreased sharply when SO
added to the CO + NO reaction system. In order to inhibit the effect of SO , substitution of Ce in the structure
of LaFeO perovskite has been investigated. It was found that La0.6Ce0.4FeO showed the maximum SO resistance
among a series of La Ce1 − xFeO3 composite oxides.
2
temperature-programmed desorption
has been investigated in this
gas was
2
2
Available online 10 March 2016
study. LaFeO
3
2
2
2
Keywords:
Perovskite
LaFeO3 catalyst
Ce
3
3
2
x
© 2016 Elsevier B.V. All rights reserved.
2
SO resistance
Selective catalytic reduction
1
. Introduction
Emissions of nitrogen oxides (NO and NO
perovskite-type oxides can select low-cost elements to replace the
sites of A or B, and be stable even above 1000 °C. Redox properties
of these oxides could be modified by the substitution of A- or B-site
cation, without destroying the matrix perovskite structure, and
oxygen vacancy will be created to meet the chemical charge valence
of the perovskite structure [7]. This vacancy could significantly
enhance the redox properties of the catalyst. Thus, it is evident
that perovskite-type oxides could be a potential catalyst for NO
reduction.
2
) in urban area have
resulted in severe environmental conditions such as acid rain and
fog and haze worldwide [1]. Reduction of nitrogen oxide to nitrogen
is the most promising process to control NO
catalytic reduction of NO by NH (NH -SCR) has been proved an ef-
fective method for the removal of NO at low temperatures [2].
However, NH is very sensitive to the reaction temperature and
x
emissions. Selective
3
3
3
atmosphere; furthermore, it is expensive and inconvenient for
industrial application. Catalytic reduction of NO by CO is a key
chemical process, because it eliminates both NO and CO emissions.
Because it uses CO as reductant, one of the usual components of
exhaust gases, this process is not affected by the possible slipping
A substantial amount of SO
exhaust gases, because of the presence of sulfur in fossil fuel.
The negative effect of SO restricts the practical application of NO
reduction. SO could form sulfide and sulfate species on the catalyst
surface, which is harmful to the catalytic reaction [8–9]. Therefore,
2
gases exists in most industrial
2
2
of NH
3
[3].
2
it is necessary to improve SO resistance of the catalysts. Ceria has
two different valences Ce3 and Ce , and is thus capable of storing
and releasing oxygen via redox shift under oxidizing or reducing
conditions [10–11]. Considering its particular redox property,
+
4+
The highly increasing demand for NO reduction, which is one of
the most important tasks for the control of air pollution, has led to
the development of new catalysts. Noble metals (e.g., Pt and Pd)
and their oxides were previously investigated as catalyst for the
reduction of NO [4–5]. However, these catalysts usually tend to
collapse the structure of the reactants at high temperatures or easily
agglomerated during the reaction. Perovskite-type mixed oxides of
A-site substitution with Ce in ABO
potential way to enhance SO resistance of ABO
In this study, LaFeO and a series of La Ce1 − xFeO
3
3
perovskite structure could be a
perovskite catalyst.
perovskite-
2
3
3
x
type oxide catalysts were prepared by citric acid sol–gel method.
All these samples were characterized by X-ray diffraction (XRD),
Brunauer–Emmett–Teller (BET) method, X-ray photoelectron
3
the general-type ABO have been widely studied for their unique
physical and chemical properties. In this particular structure, A is
the larger cation with 12-fold coordination and B is the smaller cat-
ion with six fold coordination [6]. Compared with noble metals,
spectroscopy (XPS), SO
(SO -TPD) and CO temperature-programmed reduction (CO-TPR)
to illustrate the influence of Ce substitution on SO
2
temperature-programmed desorption
2
2
resistance,
x 3
Ce1 − xFeO perovskite-
and explain the role played by Ce in La
type catalyst.
⁎
Corresponding author.
E-mail address: sunliguo411@gmail.com (L. Sun).
http://dx.doi.org/10.1016/j.catcom.2016.03.005
566-7367/© 2016 Elsevier B.V. All rights reserved.
1

5
4
Y. Qin et al. / Catalysis Communications 79 (2016) 53–57
3. Results and discussion
2
. Experimental
2
.1. Catalyst preparation
3.1. Catalytic activity results
LaFeO
3
samples were prepared by a citric acid sol–gel method. Equal
Fig. 1 shows the NO conversion rate of the La
with different amounts of Ce substitution. Fig. 1a and c shows the results
of NO conversion and N selectivity for NO + CO reaction, respectively,
with La Ce1 − xFeO samples and reaction temperature ranging from
100 to 500 °C. Without the addition of SO , the LaFeO catalyst exhibited
the maximum catalytic performance, the maximum conversion rate of
100% at 500 °C and N selectivity of approximately 80%.
Fig. 1b shows the effect of SO in NO reduction with CO. The long
x 3
Ce1 − xFeO samples
amounts of lanthanum nitrate and ferric nitrate were dissolved in
deionized water and mixed together, and then 120% (molar ratio) of
citric was added into the solution and stirred for 30 min. The solution
was evaporated at 80 °C until it became viscous. The obtained sample
was dehydrated at 120 °C for 12 h, and then calcined in airflow at
50 °C for 4 h. La
way. The proportions of lanthanum nitrate and ceria nitrate varied
according to the value of x in each La Ce1 − xFeO sample. This process
is similar to the LaFeO synthesis process, except the addition of ceria
nitrate in the first step.
2
x
3
2
3
7
x
Ce1 − xFeO
3
samples were synthesized by the same
2
2
x
3
time durable experiment started with CO + NO mixed gases, whose
temperature was maintained at 500 °C. After 30 min, a certain amount
3
of SO
catalytic activity started changing, which is attributed to the influence
of SO injection. As mentioned earlier, the LaFeO sample exhibited
the maximum NO conversion in the first 30 min; however, after the
addition of SO , the catalytic performance rapidly declined. After
approximately 100 min, catalytic conversion rate of the LaFeO sample
decreased to approximately 40–50%. When a certain amount of Ce
was introduced into the LaFeO perovskite structure, the SO resistance
of the catalysts obviously improved. In particular, the La0.6Ce0.4FeO
sample maintained a conversion rate of 80% during the latter 270 min
after addition of SO gas into the CO + NO reaction system. However,
too much Ce decreases the catalytic ability of La
and the conversion rate curve indicates that La0.2Ce0.8FeO
CeO
Fig. 1d illustrates the NO conversion curves of CO + NO reaction
with SO , O , and H O. The catalysts were tested in a mixed gas condi-
tion consisting of NO (400 ppm), CO (500 ppm), SO (100 ppm), O
(3%), and water (3 vol.%). The catalytic data curves in Fig. 1b and d indi-
2
(100 ppm) was added into the mixed gases system, and the
2
.2. Characterization
2
3
XRD patterns were acquired using a Rigaku-TTRIII diffract meter
operating at 40 kV and 40 mA with nickel-filtered Cu K radiation
λ = 1.5418 Å) in the range of 10° ≤ θ ≤ 80°, at a step size of 0.02°.
BET surface area was determined by N at 77 K using a Micrometrics
ASAP-2020 analyzer. Before each adsorption measurement, approxi-
mately 0.1 g of the catalyst sample was degassed in a N
at 300 °C for 4 h.
The CO-TPR was measured using 1% CO/Ar and 0.1 g of catalyst at
a total flow rate of 100 ml/min. Before TPR measurements, the
2
α
3
(
2
3
2
3
2
/He mixture
2
x
Ce1 − xFeO
3
catalysts,
and pure
3
catalyst was pretreated in a flow of O
2
at 500 °C for 30 min, followed
2
samples exhibited poor performance for NO reduction.
by cooling to room temperature. The catalyst was placed in a quartz
tube surrounded by a tube furnace, and the temperature was
increased to 1000 °C at a rate of 10 °C/min. The outlet gas was
monitored by Autochem2920 (Micrometrics).
2
2
2
2
2
SO
flow rate of 100 ml/min. Before TPD measurements, the catalysts were
pretreated in a flow of O at 500 °C for 30 min and then cooled to
room temperature. The samples were then treated with 1% SO /Ar for
h. The SO was purged with Ar for 1 h before starting the TPD exper-
iments. During the TPD experiments, the temperature was increased
2
-TPD was performed with 0.5 g of the catalyst sample at a total
cate that SO
La0.6Ce0.4FeO
for NO reduction and excellent SO
2
exhibits poisoning effect on NO reduction. In general,
exhibited both comparatively good catalytic performance
3
2
2
resistance to the catalytic reaction.
2
1
2
3.2. Physical properties
to 1000 °C at a rate of 10 °C/min, the SO
a thermal conductivity (TC) detector.
2
outlet gas was monitored by
Fig. 2 shows the XRD results of LaFeO
ples. The LaFeO perovskite phase (JCPDS-ICDD, 88-0641) was clearly
observed without any segregated phase, with the main peak at 32.3°
[12–13]. A single CeO phase (JCPDS-ICDD, 65–5923) was observed at
its main peak when diffraction 2θ = 28.6° [14]. For the La0.6Ce0.4FeO
sample, the substitution of Ce showed both typical LaFeO perovskite
and CeO peaks in its diffraction patterns. Peaks at 28.6° and 32.3° indi-
cated the effect of Ce substitution on the LaFeO perovskite structure.
The BET results are shown in Table 1. Pure LaFeO and CeO have
comparatively small surface areas. When Ce was doped in LaFeO
perovskite, the surface area and pore volume of La0.6Ce0.4FeO were
increased. This increase might be attributed to the substitution of Ce
in A-site of LaFeO perovskite [15]. Obviously, a larger surface of the cat-
3 3 2
, La0.6Ce0.4FeO and CeO sam-
3
XPS experiments were performed on a PHI-5300 ESCA system with Al
−8
K radiation under ultrahigh vacuum (UHV, 1.33 × 10 Pa). Before the
measurement, the sample was outgassed at room temperature in a UHV
2
3
−
7
chamber (b5 × 10 Pa). All peaks were calibrated by the carbon deposit
C 1s binding energy (BE) at 284.8 eV. The atomic ratios were calculated by
using the atomic sensitivity factors provided by the manufacturer.
3
2
3
3
2
2
.3. Activity test
3
3
Catalytic reaction was carried out in a quartz tube (d = 6 mm;
l = 60 mm), and the catalyst samples (500 mg) were placed in the
3
middle of the tube. The simulated flue gas was a mixture of NO
alyst is beneficial to the absorption and desorption of the reaction gases,
(
400 ppm), CO (500 ppm), SO
2
(100 ppm when needed), O
, while gas hourly space velocity
GHSV) = 24,000 h . Then the quartz tube was heated to increase
the temperature from 100 to 500 °C. The outlet gases NO and NO
were analyzed by MRU VarioPlus, and N O was measured by Nicolet
80. The NO conversions and N selectivity of the catalyst were
calculated by the following equations:
2
(3%),
which could enhance the catalytic performance to a higher extent.
and water (3 vol.%) with balance N
(
2
−
1
3.3. CO-TPR
2
2
Fig. 3 shows the CO-TPR results of fresh and sulfated samples of
3
2
LaFeO
3 3 3
and La0.6Ce0.4FeO . The pure LaFeO sample showed a broad
peak at approximately 700 °C, mainly attributed to the reduction of
3
+
Fe
[16]. When a certain amount of Ce was introduced into the
perovskite structure, the consumption of CO slightly increased,
LaFeO
3
NO in − NO out − NO2 out − N2O out
NO conversion ð%Þ ¼
N2 selectivity ð%Þ ¼
ꢀ 100% ð1Þ
ꢀ 100% ð2Þ
and the peak position in temperature did not change. The increase of
CO consumption might be attributed to the substitution of Ce.
In order to investigate the effect of SO on the redox property of
2
the catalyst, sulfated samples were also characterized by CO-TPR.
NO in
NO in − NO out − NO2 out − N2O out
NO in − NO out
For sulfated LaFeO samples, the TPR spectra peak moved forward to
3

Y. Qin et al. / Catalysis Communications 79 (2016) 53–57
55
Fig. 1. NO conversion of La
and (6) CeO . (a) NO conversion of La
selectivity of La Ce1 − xFeO catalysts in CO + NO reaction from 100 °C to 500 °C. (d) NO conversion of La
from 100 °C to 500 °C.
x
Ce1 − xFeO
3
samples in the reaction of catalytic reduction NO with CO. (1) LaFeO
Ce1 − xFeO catalysts in CO + NO reaction from 100 °C to 500 °C. (b) NO conversion of La
Ce1 − xFeO catalysts in CO + NO reaction with 100 ppm SO
3
, (2) La0.8Ce0.2FeO
3
, (3) La0.6Ce0.4FeO
Ce1 − xFeO catalysts at 500 °C with 100 ppm SO
, 3% O , 3 vol.% water,
3
, (4) La0.4Ce0.6FeO
3
, (5) La0.2Ce0.8FeO
3
,
2
x
3
x
3
2
. (c) N
2
x
3
x
3
2
2
approximately 600 °C, and the amount of CO consumption sharply
decreased, compared with the fresh one. This result indicated that
SO
2
resistance of the La0.6Ce0.4FeO
3
sample; its catalytic redox ability
was constant when SO
reduction system.
2
gas was added into the selective catalytic
SO
La0.6Ce0.4FeO
Compared with the fresh La0.6Ce0.4FeO
sulfated sample moved to a lower temperature region, but the amount
of CO consumption did not decrease much in the presence of SO
Thus, the results of comparison curve results illustrated an excellent
2
could inhibit the redox activity of LaFeO
sample showed different features in its CO-TPR curve.
sample, the curve peak of the
3
. However, the used
3
3
2
3.4. SO -TPD
2
.
In order to investigate the effect of SO
experiments were carried out with the LaFeO
ples. The two desorption curves are shown in Fig. 4. Two obvious
peaks were observed for the LaFeO sample. The low-temperature
peak at 300–500 °C attributed to the desorption of SO adsorbed in
the catalyst surface, and the high-temperature peak between 700 and
00 °C could be attributed to the desorption of SO adsorbed in the
subsurface layers or the interior of the catalyst [17–18]. Compared
with LaFeO , the La0.6Ce0.4FeO sample showed one significant peak at
approximately 700 °C, originated from the decomposition of sulfated
ceria species Ce(SO , and the amount of SO desorption was relatively
less. The TPD spectra of La0.6Ce0.4FeO indicated that the addition of
ceria to LaFeO perovskite inhibited the reaction with SO , which
could prevent the adverse effects of SO to the catalytic redox reaction.
This result confirmed the excellent SO resistance of the catalyst.
2
2
on the catalyst, the SO -TPD
3
and La0.6Ce0.4FeO
3
sam-
3
2
9
2
3
3
)
4 2
2
3
3
2
2
2
Table 1
Textual properties and crystallite size of different catalyst samples.
Samples
BET surface area Pore volume Pore diameter Crystallite size
2
3
(m /g)
(cm /g)
(nm)
(nm)
LaFeO
La0.6Ce0.4FeO
CeO
3
11.3
23.4
6.7
0.116
0.125
0.168
1.877
1.732
3.226
50.4
34.8
12.5
3
2
3 3 2
Fig. 2. XRD patterns of (a) LaFeO (b) La0.6Ce0.4FeO and (c) CeO samples.

56
Y. Qin et al. / Catalysis Communications 79 (2016) 53–57
Fig. 3. CO-TPR profiles of (a) LaFeO
La0.6Ce0.4FeO
3
, (b) used LaFeO
3
, (c) La0.6Ce0.4FeO
3
, (d) used
3
Fig. 5. XPS results of Ce 3d over La0.6Ce0.4FeO sample.
3
.
provided by the manufacturer. Ce4+ tends to absorb SO
Ce(SO , which could inhibit the poisoning effect of SO to the LaFeO
3
and form
2
3
.5. XPS analysis
Ce often appeared as a crucial component for catalysts in selective
)
4 2
2
perovskite [20]. Combined with the catalytic activity results, the
presence of Ce4 on the catalyst surface could increase the catalytic
+
catalytic reduction, because of its unique redox property. The valence
x 3
activity of the perovskite structure La Ce1 − xFeO .
state of the specific element is important to the catalytic reaction. Ce
has two different valence states Ce4 or Ce . Therefore, XPS experi-
+
3+
4. Conclusion
ments were performed to investigate the surface component and chem-
ical state change of Ce over the La0.6Ce0.4FeO
3
sample. The XPS curves
LaFeO perovskite oxides were prepared and found to exhibit appre-
3
are shown in Fig. 5, and the peaks are fitted by Gaussian–Lorentz curves.
The Ce 3d peaks were fitted into sub-bands by searching for the opti-
mum combination of Gaussian bands. The multiple spectra of Ce 3d
were decomposed into eight components by the Gaussian–Lorentz
fitting procedure. The bands labeled u were due to 3d3/2 spin orbit states
and those labeled v represented 3d5/2 states. The bands labeled as u1
ciable catalytic performance in NO reduction. Their catalytic activity de-
cline sharply when SO gases were added into the CO + NO reaction
2
system. The SO resistance of the catalyst was significantly increased
2
3
3
after Ce substituting into the A-site of the LaFeO perovskite. Because
of the special redox property of Ce, the La0.6Ce0.4FeO catalyst could ab-
sorb SO
2
without affecting its catalytic activity. According to the XRD
3
+
and v1 were attributed to the primary photoemission of Ce and the
other six bands labeled as u2, v2, u3, v3, u and v were attributed to
patterns of catalyst samples, combined with the CO-TPR and SO -TPD
2
results, La0.6Ce0.4FeO
while still exhibiting comparatively good reductive activity and remark-
able SO resistance. Thus, the substitution of Ce is the key factor to the
improvement of SO resistance in LaFeO perovskite catalysts.
3
maintains the original perovskite structure,
4
+
3+
4+
Ce
[19]. The fitting results revealed that both Ce
and Ce
exist
in the sample. From the peak areas of each state, Ce in the catalyst is
mainly in the valence state of +4, and the corresponding results of
2
2
3
4
+
3+
4+
Ce /Ce + Ce ratio 82.4% were calculated by using the program
Acknowledgments
This study was supported by the Project of Low Concentration Sulfur
Dioxide Flue Gas Treatment under Grant No. 738010004. The authors
are grateful for the support of the Central South University and the
valuable technical help of Dr. He.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.03.005.
References
[
[
[
1] Mengmeng Li, Vencon G. Easterling, Michael P. Harold, Appl. Catal. B. 184 (2016)
64–380.
2] Dimitrios K. Pappas, Thirupathi Boningari, Punit Boolchand, Panagiotis G. Smirniotis,
J. Catal. 334 (2016) 1–13.
3] Ping Xiao, Ryan C. Davis, Xiaoying Ouyang, Jinlin Li, Arne Thomas, Susannah L. Scott,
Junjiang Zhu, Catal. Commun. 52 (2014) 69–72.
3
[4] C.N. Costa, A.M. Efstathiou, J. Phys. Chem. C 111 (2007) 3010–3020.
[5] D.I. Kondarides, T. Chafik, X.E. Verykios, J. Catal. 191 (2000) 147–164.
[6] M.A. Peña, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981–2017.
[7] C.H. Kim, G.S. Qi, K. Dahlberg, W. Li, Science 327 (2010) 1624–1627.
[8] Z.B. Wu, R.B. Jin, H.Q. Wang, Y. Liu, Catal. Commun. 10 (2009) 935–939.
Fig. 4. SO
2
-TPD profiles of the (a) LaFeO
3
and (b) La0.6Ce0.4FeO
3
samples.
[9] R.Q. Long, R.T. Yang, J. Am. Chem. Soc. 121 (1999) 5595–5598.

Y. Qin et al. / Catalysis Communications 79 (2016) 53–57
57
[10] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, J. Phys. Chem. B
[16] B.P. Barbero, J.A. Gamboa, L.E. Cadús, Appl. Catal. B. 65 (2006) 21–30.
107 (2003) 5162–5167.
[17] C. Yanxin, J. Yi, L. Wenzhao, J. Rongchao, T. Shaozhen, H. Wenbin, Catal. Today 50
[
[
11] L. Chen, J. Li, M. Ge, J. Phys. Chem. C 113 (2009) 21177–21184.
12] S. Thirumalairajan, K. Girija, V. Ganesh, D. Mangalaraj, C. Viswanathan, N.
Ponpandian, Cryst. Growth Des. 13 (2013) 291–302.
(1999) 39–47.
[18] Dong Wook Kwon, Ki Bok Nam, Chang Hong Sung, Appl. Catal. B. 166–167
(2015) 37–44.
[
[
13] M. Daturi, G. Busca, Chem. Mater. 7 (1995) 2115–2126.
14] J.H. Chen, M.Q. Shen, X.Q. Wang, J. Wang, Y.G. Su, Z. Zhao, Catal. Commun. 37 (2009)
[19] W. Shan, F. Liu, H. He, X. Shi, C. Zhang, Appl. Catal B. 115–116 (2012) 100–106.
[20] S. Yang, Y. Guo, H. Chang, L. Ma, Y. Peng, Z. Qu, N. Yan, C. Wang, J. Li, Appl. Catal B.
136–137 (2013) 19–28.
105–108.
[15] J.J. Zhu, X.G. Yang, X.L. Xu, K.M. Wei, Sci. China Ser. B 50 (2007) 41–46.